1. Field of the Invention
The present invention relates to a device for spraying coating onto a pipe. Specifically, the invention relates to a device having a yoke which can be releasably mounted around a pipe or pipeline, and once mounted, the device travels longitudinally along the pipeline spraying a continuous layer of coating onto the exterior surface of the pipe. The device applies the coating in a helical pattern utilizing spray guns which move on the yoke in opposite reciprocating arcs of approximately 180 degrees, with each spray gun spraying only while moving in one direction, i.e. either clockwise or counterclockwise, on the yoke.
2. The Prior Art
Preliminary searches were conducted on the invention disclosed herein, and the following patents were uncovered in the searches:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 2,535,451 Phillips Dec. 26, 1950 2,583,819 Cummins Jan. 29, 1952 2,613,635 MacWilliam Oct. 14, 1952 3,107,183 Way Oct. 15, 1963 3,386,414 Faber June 4, 1968 3,654,898 Galitz Apr. 11, 1972 4,038,942 Hart Aug. 2, 1977 ______________________________________
Many spray coating devices operate by coordinating a moveable spray gun on the device with a moveable object to be coated. An example of this type of device can be found in U.S. Pat. No. 3,654,898 where the spray gun moves linearly in the direction opposite to the direction of travel of the object to be coated. Such a device is limited to use with small objects which can be moved easily and is not appropriate for coating larger objects, such as pipelines, where moving the object is impractical, if not impossible.
A variation of this initial concept is to permanently mount spray guns on a moveable yoke, and then move the yoke longitudinally along the object to be coated. Such a design can be found in U.S. Pat. No. 2,535,451 which discloses a device with multiple spray guns mounted around a yoke for coating smoke stacks, grain elevators, and the like. This design has several problems when employed to coat a horizontal pipeline.
First, the large number of spray guns on the yoke increase the probability that a spray gun will become clogged which, if not corrected, would create a break in the coating. On a pipeline, such a break in the coating would expose the uncoated strip on the pipe to corrosion and result in reduced service life of the pipe.
Even if the clogged spray gun was promptly discovered, correcting the break in the coating would be time consuming, particularly because the pipe normally must be inductively preheated immediately prior to applying the coating and the coating device is not designed to spray only one strip of coating onto the pipe. Therefore, it would be difficult to keep the device properly positioned in order to create the proper spray overlap with the previously coated section of the pipe.
Second, the permanent mounting of the spray guns on the yoke limits the flexibility to adjust overlap of spray between the spray guns and limits control of the thickness of the coating being applied. To insure adequate coverage, sufficient overlap must be provided between the spray guns. Adjacent spray guns spray coating on the surface simultaneously in the overlap regions and overlap regions which are horizontal in relationship to the pipe create the problems of solvent inclusion and runs in the coating. Solvent inclusion prevents the coating from drying properly and runs cause non-uniform coating thicknesses.
Drips resulting from application of excess coating in the overlap regions are also expensive due to the cost of wasted coating material. U.S. Pat. No. 2,583,819 provides a drip pan designated by numeral 77 for catching and recycling coating which drips off the pipe. Although recycling of coating is a way to reduce the cost of some types of coating materials, it is not possible to recycle coatings which are formed by mixing two components at the spray gun where those components set up quickly after being mixed and sprayed onto a heated pipe.
The remaining patents listed above are not considered sufficiently pertinent as to require any comment.
The present invention is superior to the prior art in that it is lighter in weight, easier to position on the pipe, and has fewer spray guns to clog. Further, the present invention has greater flexibility than the prior art in that the thickness of coating can be controlled and adjusted by varying the travel speed of the device, varying the speed of the spray guns, and varying the width of the spray pattern applied by the spray guns.
The invention's operation which provides closer control capability and a time interval between application of spray patterns on the pipe tends to minimize the problem of solvent inclusion. Also these same factors, in conjunction with spray overlap regions which are diagonal in relationship to the pipe, tend to eliminate the problems of runs and drips in the coating.